1. Field of the Invention
The present invention relates to friction clutches with a low declutching force, especially for motor vehicles, and relates more particularly to the clutch mechanism which is included in such a clutch.
2. Description of the Related Art
As is known in a conventional clutch, a diaphragm interposed between a driving shaft and a driven shaft bears on the base of a cover plate, which is fixed to a rotary driving flywheel, so as to displace a pressure plate towards the said flywheel, which constitutes a reaction plate, in order that the friction liners of a clutch friction wheel shall be gripped between the pressure plate and reaction plate.
The driving flywheel is fixed to a first shaft, for example a driving shaft, while the clutch friction wheel has at its inner periphery a hub for mounting it in rotation on a second shaft such as a driven shaft.
The clutch is accordingly normally engaged with torque transmission between the driving and driven shafts.
In order to disengage (declutch) the clutch, it is necessary, with the aid of a clutch release bearing, to exert an annular thrust, in the case of a clutch of the push-to-release type, on the inner ends of the fingers of the diaphragm so as to cause the said diaphragm to pivot and overcome the force exerted by the diaphragm on the axially movable pressure plate, whereby to release the friction liners.
The torque is then no longer transmitted from the driving shaft to the driven shaft, because the friction liners are no longer gripped between the pressure plate and the reaction plate which are fixed to the cover plate in rotation.
Typically, the cover plate, the pressure plate and the diaphragm constitute a unitary assembly, referred to as a clutch mechanism, which can be fitted through its cover plate on the flywheel, with elastic tongues coupling the pressure plate to the cover plate for rotation with axial mobility.
As is known, the diaphragm has a central hole and includes an outer peripheral portion in the form of a Belleville ring which is extended internally by a central portion divided into radial fingers by slots. The diaphragm bears, through its Belleville ring portion, on the cover plate and on the pressure plate. Thus, the declutching means for the clutch, which enables the action of the resilient clutch engaging means to be counteracted at will, consists of the fingers of the diaphragm, on the inner ends of which the clutch release bearing acts, while the Belleville ring of the diaphragm constitutes axially acting resilient clutch engaging means for causing the friction liners to be gripped between the pressure plate and the reaction plate, and therefore for axially urging the pressure plate in a direction away from the base of the cover plate.
In the free state, the diaphragm is frusto-conical in form. Once it has been mounted in the clutch, its Belleville ring is mounted under prestress, and is flattened to a greater or lesser extent. During operation of the clutch (i.e. disengagement of the clutch), the cone angle of its Belleville ring is modified.
As is known, the characteristic curve of this diaphragm (with force exerted plotted against deflection), for example taken at the level of the inner ends of the fingers of the diaphragm, is determined by the dimensions of its Belleville ring and by the ratio between the depth of the frustum of the cone of the Belleville ring in the free state and the thickness of the diaphragm.
This characteristic curve passes through a maximum.
Thus, the force to be exerted on the inner ends of the fingers of the diaphragm increases to a maximum, diminishes gradually down to a minimum, and then rises again.
The difference between the maximum and the minimum may be large. For more detail as to this characteristic curve, reference should be made for example to the documents FR-A-1 392 569 or WO97/19275, and also to FIGS. 4 and 5 which show respectively the depth H of the frustum of the cone of a Belleville ring and the said curve, in which the load it exerts is expressed on the ordinate, and its deflection on the abscissa.
In the document FR-A-1 392 569, an elastic progressive engagement device was provided so as to avoid passing through the above mentioned maximum during the declutching operation. This device is mounted in series with the Belleville ring of the diaphragm, and exerts an elastic force which is significantly smaller than that of the Belleville ring. The device has a course of travel which is limited between a position of prestress in which its force is a maximum, and a stressed position in which its force is a minimum.
With this arrangement, a generally increasing declutching force is obtained at the clutch release bearing.
During the declutching operation, it is not possible to obtain as great an assisting force as desired, because the progressive engagement device reduces the load exerted by the diaphragm on the pressure plate when the clutch is engaged.
The intervention of an auxiliary spring in parallel with the diaphragm may accordingly be considered.
In that case, the auxiliary spring, for example in the form of a Belleville ring, may bear on the cover plate and on the inner ends of the fingers of the diaphragm.
When the clutch is in its engaged position, it can therefore exert a minimum force, and then, as its conicity varies, exert an assisting force during the declutching operation. It is then necessary that this assisting force shall be at a minimum, in the engaged condition of the clutch, over the whole working life of the clutch.
The ideal for this type of embodiment is to make use of a regulating device or so-called wear compensator, which maintains the diaphragm always in generally the same position regardless of the wear in the friction liners of the clutch and/or the friction surfaces of the pressure and reaction plates, in order that the auxiliary spring shall, under all circumstances throughout the working life of the clutch, exert a very low force when the clutch is in its engaged condition. To obtain an assisting force is therefore not as simple as is desirable.
An object of the present invention is to overcome these drawbacks, and accordingly to provide, in a simple and inexpensive way, a friction clutch with a low declutching force which makes use of springs acting in series, but without, at any time in the working life of the clutch, diminishing in particular the gripping force on the friction liners when the clutch is engaged.